1. Technical Filed
The present invention relates to a backlight module, more particularly to an edge-type backlight module used in a liquid crystal display (LCD) device.
2. Discussion of the Related Art
Liquid crystal display devices (LCDs) have been widely used as displays for notebook computers, flat-screen TVs, computer monitors, cell phones, personal digital assistants (PDAs), digital cameras, and the like. With the need for an effective backlight module for LCDs increasing, the trend for the backlight module is a lighter weight, a thinner size, a brighter illumination, a more integration, a lower fabrication cost, and a reduced power consumption. A conventional backlight module can be divided into two types, i.e., a direct type and edge type, according to the location of the light source. An edge-type backlight module in which the light source is located facing a light input surface that orthogonally adjoins a light emission surface in a given light guide plate is widely used in LCD devices. Light beams emitted from the light source are optically coupled into the light input surface, transmitting into the light guide plate, are ably reflected by the microstructure of a reflective back surface, and eventually then transmitted out from the light output surface uniformly to illuminate an LCD panel.
Referring to FIG. 8, a conventional backlight module 10 includes a light source 11, a light guide plate 13, a reflective plate 12, a diffusion plate 14, a prism sheet 15, and the like, wherein the light guide plate is made of a material selected from a group consisting of polymethyl methacrylate (PMMA), polycarbonate (PC), and any other suitable transparent resin material. The function of the reflective plate 12 is to reflect light beams that are not diffused back to light guide plate. The function of the diffusion plate 14 is to diffuse light beams to therefore eliminate a light column zone formed by the microstructure of the light guide plate. The function of the prism sheet 15 is to collect light beams and therefore improve the illumination.
The backlight module 10 can use, e.g., one or more cold cathode fluorescent lamps (CCFL) or a light emitting diodes (LED) as the light source 11. A small-sized backlight module usually uses at least one LED as light source and a large-sized backlight module usually uses a CCFL as light source. The function of a light guide plate is to transform a line source to a plane source when using a CCFL as the light source. However, the function of a light guide plate is to transform a point source to a plane source when using a LED as the light source. Advantages of LED over CCFL include the following. First, the LED has a long life, a bright color, and a high reliability. Second, the LED is not harmful to environment unlike the CCFL, which potentially is because of mercury in the fluorescence tube thereof. So, it may be a development trend that LEDs are used as the preferred light source of an edge-type backlight module.
Referring to FIG. 9, in which the backlight module employs an LED light source, a number of light zones 22 may be occur in areas adjacent to the light sources, and a number of dark zones 24 may appear between two given light zones 22. The light zones 22 and the dark zones 24 occur due to the light column phenomenon. This phenomenon reduces light distribution uniformity.
Referring to FIGS. 10 and 11, in order to solve the above problems, a sawtooth prism structure 34 or a v-cut groove structure 44 is formed on a portion of the light input surface 32 facing each of the light sources. Light beams pass through the sawtooth prism structure 34 and the v-cut groove structure 44 and then are diffused. In the configuration, the light column phenomenon may be eliminated at a certain degree. However, a portion of light beams may be reflected by the sawtooth prism structure 34 and/or the v-cut groove structure 44, so the light beams cannot transmit into the light guide plate. As such, these structures can reduce a utilization efficiency of light beams.
FIG. 12 is a schematic view showing that light beams are diffused by the microstructure of the light input surface of FIG. 10. The light beams are diffused by the microstructure of the light input surface of FIG. 11, which is similar in configuration with that of FIG. 10. According to the Fresnel formula, the refractive angle can be achieved according to the following arithmetic expression:
  β  =      90    -          α      /      2        -          arcsin      ⁡              (                              sin            ⁡                          (                              90                -                                  α                  /                  2                                            )                                n                )            In that expression, β is a refractive angle, n is refractive index of the light guide plate, and α is an apex of the microstructure of the light input surface. It is shown that the dark zone cannot be eliminated completely because of a limited refractive angle (i.e., the max refractive angle is less than 50 degrees, for the light guide plate made of PMMA).
Referring to FIG. 13, another conventional back light module discloses that a number of LEDs 64 is surrounded by a number of reflectors 66 for eliminating the light column phenomenon. Each of the reflectors 66 respectively has a circle-arc section and is opposite to a corresponding LED 64. An emitting surface 642 of the LED 64 is arranged facing the reflector 66, with its back to a light input surface 622. This configuration can eliminate partly the light column phenomenon. However, the LED 64 also can shield partly the light beams reflected by the circle-arc shape reflector 66, and a dark zone can be formed on areas of the light guide plate distant from the LED 64. This configuration tends to reduce the light distribution uniformity and utilization efficiency of light energy.
Therefore, what is needed is to provide a backlight module to increase illumination uniformity, reduce the light column phenomenon, and improve light distribution uniformity and utilization efficiency of light energy.